#include <bits/stdc++.h>
using namespace std;
int n;
int sum;

int main() {
    cin >> n;

    int m = 1;
    int t = 0;
    for (int i = 1; i <= n; i++) {
        sum += m;
        if (++t == m) {
            m++;
            t = 0;
        }
    }
    cout << sum;
    return 0;
}